Screw extruders have been employed for various purposes including applications in plastics technologies. Such devices utilized auger screws for transporting as well as extruding the working material. Twin screw extruders included a pair of screws extending along parallel axes relatively close to one another. To assure uniform extrusion flow, the screws were required to be driven both at the same speed. In some instances extruder screws were driven in the same direction of rotation, while in other instances each extruder screw was driven in an opposite direction. This, of course, depended upon the screw configuration. Relatively high torques were required to drive the screws. Such applications presented unique requirements for screw drive mechanisms.
One approach at providing a drive mechanism for a twin screw extruder was illustrated in United Kingdom Pat. No. 1,421,704. The extruder drive disclosed in such patent included a pair of output shafts, each of which was coupled to an extruder screw.
Each output shaft was driven by a planetary gear train having a separate motor. Speed synchronization was achieved by a gearing arrangement between the output shafts of each motor.
Each planetary gear train included a ring gear which was mounted for rotation within a drive casing by a ball bearing. Each gear set further included an idler sun gear and two planet gears, one of which was fixed to an input shaft and the other to an output shaft. The planet gears of each gear set were offset from each other about 90 degrees along the ring gear. One planet gear was driven through the input shaft by the motor. The other planet gear was fixed to the output shaft and was driven by the relative rotation between the sun gear and the ring gear. The two gear sets were coaxial but axially spaced from one another with the input shaft of one gear set being longer than the input shaft of the other.
Among the problems encountered with such prior drive system was that it was relatively bulky, heavy, and suffered from uneven distribution of loads over the ring gears. Excessive transverse forces acted upon the ring gears which mandated relatively heavy bearings for mounting the ring gears within the casing.
Additionally, because the power transmission from the input shaft to the output shaft extended through only two planet gears, relatively wide gearings were required to increase power transmission, increasing the weight, size and cost of the drive system.
A further problem encountered with the prior drive system was that since speed synchronization between the two gear sets was accomplished through a mechanical gear engagement, drive power from the motors were diverted to speed compensation functions rather than utilized solely for driving the extruder screws.